This command will search for directories in the current directory and its subdirectories, find the oldest subdirectory in each directory, and display them on to screen:
find . -maxdepth 1 -type d -exec sh -c 'oldest_folder=$(ls -ltr "$1" | awk "/^d/ {print \$NF; exit}"); echo "$1/$oldest_folder"' sh {} \;
To truncate all files in those oldest subdirectories (snap.X on the screenshot above) use:
find . -mindepth 1 -type d -exec sh -c 'oldest_folder=$(ls -ltr "$1" | awk "/^d/ {print \$NF; exit}"); > "$1/$oldest_folder"/*' sh {} \;